Japanese Laid-Open Patent Publication No. H09-266055 discloses one example of spark plug, which includes: a cylindrical insulator; a center electrode held in the insulator; a ground electrode defining a spark gap with the center electrode; and a resistor disposed on a rear end side of the center electrode within the insulator so as to suppress radio noise generated from the center electrode. In this spark plug, a conductive seal material such as glass seal material is arranged between the resistor and the center electrode in contact with an outer circumferential surface of the resistor so as to ensure electrical conduction to the center electrode while providing an improved seal on the inside of the insulator.
In the above-disclosed type of spark plug, a difference in thermal expansion coefficient between the insulator and the center electrode tends to be large so that a clearance may occur between the center electrode and the conductive seal material due to such a difference in thermal expansion coefficient during the manufacturing of the spark plug.
In the case where the conductive glass seal material is fixed between the center electrode and the resistor by hot pressing, for example, the conductive glass seal material (in the form of a raw material powder) is placed between the center electrode and the resistor in a thorough hole of the insulator, melted under heating and then solidified under cooling. However, the amount of thermal shrinkage of the center electrode during the cooling becomes larger than that of the insulator as the difference in thermal expansion coefficient between the center electrode and the insulator increases. This makes it likely that a clearance will occur at a location adjacent to an interfacial surface of the center electrode. The higher the occupation rate of the center electrode in the through hole of the insulator, the more likely the influence of the difference in thermal expansion coefficient will occur. The influence of the difference in thermal expansion coefficient is of particular concern in the vicinity of a rear-end-side collar portion of the center electrode.